I am not an expert in ServiceNow. I used all information here including every commenter's script and screenshot. 

Following Malani Arjun's Screenshot, the include script is wrong. I wasted 3 hours solving this. 🤣

Conclusion:  

Remove this code that is automatically injected by ServiceNow when you create new Include Script. 

var renderMailtoButton = Class.create();
renderMailtoButton.prototype = {
initialize: function() {
},

type: 'renderMailtoButton'
};

Just directly insert below into the included script.

function renderMailtoButton(image, response, emailbod){
	
	var instance = gs.getProperty("instance_name");
	var link = "https://" + instance + ".service-now.com";
	var mark = email.watermark;
	var number = current.number;
	var emailAddress = instance + "@service-now.com";
	if (current.getTableName().indexOf("sysapproval") != -1){
		number = current.sysapproval.number;
		}
			var mailLink = '<a href="mailto:'+ emailAddress + '?subject=Re:' + number + ' ' + response +'&body=' +  emailbod +  mark + '"><img src="' + link + image + '"></a>';
	return mailLink;
	
}